Liquid crystal displays (LCDs) take up an important position in information display technologies. An LCD displays images by applying electricity to both substrates with a liquid crystal layer therebetween. Because an LCD panel itself cannot emit light, the LCD needs help the aid of a backlight to generate light.
The backlight is directly associated with brightness and power consumption of the LCD panel. Furthermore, the backlight is one of the most expensive parts of a display module. Accordingly, saving of backlight-related costs may have a crucial effect on the LCD demand. As a light source for a backlight unit, CCFLs, EEFLs, FFLs, or LEDs have been employed. A light source employing LEDs gain popularity with advantages of rapid response speed, good color gamut, and eco-friendly nature in comparison with CCFL light sources.
FIG. 1 is a cross-sectional view illustrating a white LED backlight unit according to the related art. Referring to FIG. 1, the white LED backlight unit includes a white LED light source 100, a light guide 200, a reflection plate 300, and a diffusing plate 400. Specifically, light emitted from the white LED light source 100 is reflected upward by the reflection plate 300. Then, reflected light passes through the light guide 200 and is diffused by the diffusing plate 400, so that white light 40 is radiated. The “white LED” means an LED emitting white light. As methods of implementing a white LED, there are a single chip method and a multichip method. In the single chip method, a blue or violet LED chip is combined with a phosphor to produce a white LED. In the multichip method, two LEDs whose colors are in a complimentary relation or different colors of LED chips are combined to embody a white LED. A white LED may also be implemented using a blue LED emitting blue light together with a phosphor.
FIG. 2 is a cross-sectional view illustrating a white LED light source using a phosphor according to the related art. Referring to FIG. 2, a white LED light source 100 using a phosphor according to the related art includes a frame 120 and a blue LED 110 that is mounted in the frame 120 and buried in a high molecular material, such as, for example, a polymer resin. A phosphor 10, for example, a YAG-based phosphor radiating yellow light (whose wavelength is about 560 nm) is contained in the high molecular material. As a result, blue light 20 emitted from the blue LED 110, which is a nitride-based semiconductor device, is partially absorbed by the YAG-based phosphor 10 which is in turn excited to emit yellow light. The blue light and the yellow light are mixed to produce white light 40. This method has advantages, such as low costs and a simple power circuit, but suffers from a low emission efficiency. Further, blue light 20 and yellow light 20 both have broad wavelength intervals, and this may cause color separation so that strobbing may be prone to occur. Accordingly, it is difficult to mass produce white LEDs having the same color coordinates.